On board on a vehicle, such as a truck, a bus or a working vehicle, air for use in for instance the brake system and/or the wheel suspension system of the vehicle is compressed. For pressurizing the air a compressor is used, which frequently is lubricated with oil of the same kind that is used for the lubrication of the engine by which the vehicle is driven. The compressor is driven by means of said engine and supplies intermittently, when so needed, compressed air to one or several overpressure containers in the vehicle. From these containers, compressed air is delivered to for instance the brake system of the vehicle when so required.
Compressed air produced in this manner contains suspended particles of oil, water and a plurality of solid particles including hydrocarbon compounds. The solid particles are formed as a consequence of a relatively high temperature in the compressor. In order to be able to use the brake system and the suspension system of the vehicle the compressed air has to be relieved from on one hand the main part of the particles and on the other hand from vaporised water.
Traditionally, compressed air, which is produced on board on a vehicle, is cleaned by being brought to pass through a filter. In the filter, the plurality of solid and liquid particles are caught, but also vaporised water is separated in a part of the filter, which includes a bed of material especially intended for this purpose. Usually, the compressed air is cooled before it is cleaned so that certain of the vaporised substances are condensed. In certain cases the cooled compressed air is also brought to pass through a so-called pre-separator, for instance a cyclone, for being relieved from the largest of the suspended particles.
For reconditioning of the filter, the latter is back-flushed intermittently with a part of the compressed air that has passed through the filter, wherein a filtrate and moisture absorbed in the filter is removed from the filter and conveyed out of the compressed air system of the vehicle.
In spite of a repeated reconditioning of the filter material, this has to be replaced after a certain operational time period since it is clogged by oil and solid particles which can not be removed completely by the back-flushing with cleaned compressed air. Since the filter material is relatively expensive, it is desirable to replace the same as seldom as possible. Furthermore, it is also an evident desire to reduce further costs for the replacement of the filter bed.